Wavelength Dependence of Frequency-Response Measurements in Multimode Optical Fibers

Wavelength Dependence of FrequencyResponse Measurements In Multimode Optical Fibers By L. G. COHEN, H. W. ASTLE, and I. P. KAMINOW (Manuscript received May 24, 1976) A newly developed technique for directly measuring fiber dispersion in the frequency domain as a function of wavelength is described. Spectrally filtered white light from a xenon arc lamp is sinusoidally modulated in the range 0 to 1 GHz by an electrooptic modulator and injected into a fiber. The procedure is to vary the modulation frequency and measure the corresponding sideband output power with a photo multiplier and spectrum analyzer. Ratio measurements between the test fiber and a short manium- and boron-doped fibers have been examined. The least dispersive borosilicate graded-index fiber has a 1 dB bandwidth of 1 GHz, after 1.07 km of propagation at X = 908 nm. The width broadens gradually icith increasing wavelengths up to X = 1100 nm. I. INTRODUCTION reference fiber give the baseband frequency response. A number of ger- Optical fiber waveguides are potentially useful for transmitting analog signals as well as digital pulses in communication systems. The information-carrying capacity of such a waveguide is determined by its impulse response in the time domain or equivalently in the frequency domain by the spectral transfer function, which is the Fourier transform of the impulse response. Most of the previous studies1 of fiber dispersion have analyzed the fiber response to short laser pulses. In these studies we are limited to wavelengths for which pulsed laser sources are available.